Prior art cast draft sills are constructed to replace many of the mechanical elements in a draft sill structure that are necessary to transmit the draft and buff forces from the railroad car coupler to the car's center sill. However, these prior art cast draft sills are not constructed so that the center plate can be replaced without either replacing the entire cast draft sill or replacing the center plate with a conventional fabricated style bolt-on center plate. Also the prior cast draft sills are sometimes fabricated in segments with one segment combining the striker, front draft lug and key slot and the other segment combining a center brace, rear draft lugs, center plate and king pin mounting hole. An obvious difficulty with the separate prior art castings is that they must be precisionally aligned and welded to the railroad car center sill. Another not so obvious problem arises when considering distribution of the buff and draft forces through the structure from the coupler to the railroad car's center sill. In this respect, the load carrying members of the structure must be designed to distribute these forces and avoid extreme stress concentrations that inherently shorten the useful life of the railroad car.
Those prior art cast draft sills which incorporate the center plate and king pin hole into the unitary structure must provide substantial bracing for this portion of the structure due to the necessity to transmit vertical forces generated by the weight of the car, the lading, the rock-and-roll motion of the car, and vertical accelerations of the car from the car body bolster to the car's truck and wheel set. These forces must be transmitted through the casting and particularly the center plate portion thereof at the lowest stress possible and with a minimum of stress concentrations. Also this cast draft sill must remain in the same geometric relation with mechanical elements of the car as between their fabricated counterparts in the standard AAR (American Association of Railroads) draft sill arrangement.
Incorporation of a removably mountable center filler plate offers many advantages but necessitates an inventive consideration for it to be utilized at a unitary cast sill structure. Advantages of a removable center filler plate are its replaceability for repairing or rebuilding a railroad car and that a single casting can be fitted with center plates of a different bearing surface diameter as required by the particular application. Adapting a unitary cast center sill for use with a removable center filler plate requires that a rectangular pocket be provided in the center sill for mounting the center filler plate and the four sides of the pocket being elements of a four-sided column with the bottom element being the center plate which must be a rigid compression member. Vertical forces from the car are applied to the four sides of the column by a combination of structural members in the car body bolster and the center sill. These vertical forces are transmitted downward through the walls of this column and through the center filler plate to the truck bolster. The two sides of the center filler plate pocket that are transverse to the longitudinal axis of the car are formed by an inboard end wall and an internal wall of the casting and these are designed to carry high shear loads that result from rock-and-roll actions of the car. These shear loads undergo a full reversal during each cycle as the car rocks and these transverse walls resist the tendency of the center sill to be deformed to a "parallelogram" shape due to the load.
The sides of the draft gear pocket must carry the buff and draft force loads as well as vertical forces generated by the rock-and-roll motion. The two sides of the draft gear pocket that are arranged parallel to the longitudinal axis of the car are typically vertical webs of the Z-sections that comprise the center sill of the car. These vertical webs carry a major share of the buff and draft loads that are applied to the front and rear draft lugs. A problem with this portion of the prior structures is a tendency of the rear draft lugs to collapse inward from buff loads applied to them by the draft gear. Because of the incorporation of the center sill pocket, it is not possible to distribute these forces over the same load path as in previous cast draft sills without the center filler plate pocket. In these prior structures, these forces were transmitted into ribs behind the rear draft lugs and on into the side walls of the sill through shear. These ribs in prior draft sills extended in an inboard direction through the center plate and king pin portion of the structure and are substantially longer than ribs that could be utilized when providing a pocket for the described center plate. In overcoming this deficiency, the cast draft sill of this invention incorporates a U-shaped beam appropriately placed for distributing the loads and permitting formation of the center filler plate pocket.
In distributing loads from the inboard end of the prior art cast draft sills, they use a direct connection to the railroad car center sill without accounting for load distribution from the cast draft sill into the car center sill due to different tensile and compressive forces occurring at this juncture. To alleviate a condition of high strain sections meeting low strain sections of the structure, the present cast draft sill incorporates a transition element to join the cast draft sill in the car center sill. This transition element includes a fillet plate referred to as a "fish-tail plate" that is formed as a part of the casting and PG,5 coupleable with the railroad car center sill in a telescopic fashion. The transition element also includes a lip around the sides and bottom of the casting to aid in load distribution.